This invention relates to an Fe-B-R type rare earth permanent magnet having high magnetic properties. (In the present invention, R represents the rare earth elements inclusive of Y) More particularly, it is concerned with a permanent magnet based on rare earth element (R), boron (B) and iron (Fe), with its corrosion resistant property being improved significantly by the particular compositional ratios of the constituent elements.
There was previously proposed by three of the present inventors, as an improved permanent magnet of high performance which exceeded the highest magnetic properties of the conventional rare earth-cobalt magnet, an Fe-B-R type permanent magnet which was composed of as the principal components iron (Fe), boron (B) and light rare earth elements such as neodymium (Nd) and praseodymium (Pr) abundantly available in the natural resources, but not using samarium (Sm) and cobalt (Co) which are scarcely available in the natural resources or uncertain in the commercial availability, hence expensive (Japanese Patent Kokai Publications No. 59-46008 and No. 59-89401 or EPA 101552).
Said inventors also succeeded in obtaining another Fe-B-R type permanent magnet having a higher range of the Curie temperature than that of the abovementioned magnetic alloy which ranges, in general, from 300.degree. C. to 370.degree. C., by substituting cobalt (Co) for a part of iron (Fe) (Japanese Patent Kokai Publications No. 59-64733 and No. 59-132104 or EPA 106948).
With a view to improving the temperature characteristics (in particular the coercivity "iHc"), while retaining the Curie temperature equal to, or higher than that, and a higher (BH)max than that, of the above-mentioned Co-containing Fe-B-R type (i.e., more precisely (Fe,Co)-B-R type) rate earth permanent magnet, use said inventors further proposed still another Co-containing Fe-B-R type rare earth permanent magnet with much more improved iHc, while still retaining a very high (BH)max of 25 MGOe or above, which could be realized by including at least one kind of heavy rare earth elements such as dysprosium (Dy), terbium (Tb), etc. as a part of R of the Co-containing Fe-B-R type rare earth permanent magnet, R mainly containing light rare earth elements such as Nd and/or Pr (Japanese Patent Kokai Publication No. 60-34005 or EPA).
However, the permanent magnets having the abovementioned excellent magnetic properties and being composed of the Fe-B-R type magnetically anisotropic sintered body contain, as its principal constituents, those rare earth elements and iron which are apt to be oxidized in the air and tend to gradually form stable oxides. On account of this, when such permanent magnet is assembled in the magnetic circuit, various problems and inconveniences would be brought about by the oxides formed on the surface of the magnet: such as decrease in output of the magnetic circuit; irregular functioning among the magnetic circuits; and, in other aspect, contamination of various peripheral devices around the magnetic circuits due to scaling off of the resultant oxides from the surface of the magnet.
In order therefore to improve the corrosion resistant property of the abovementioned Fe-B-R type permanent magnet, there was already proposed a permanent magnet with an anti-corrosive metal layer having been plated on its surface by the electroless plating method or the electrolytic plating method (Japanese Patent Application No. 58-162350), and another permanent magnet with an anti-corrosive resin layer having been coated on its surface by the spraying method or the dipping method (Japanese Patent Application No. 58-171990).
With this plating method, however, there still remained problem such that, since the permanent magnet is a sintered, somewhat porous body, an acidic or alkaline solution used for its pre-treatment before the plating procedure stays in the pores of the sintered magnet body, which is apprehensively liable to corrode the magnet with lapse of time; and further, since the magnet body is inferior in its chemical-resistant property, the surface of the magnet is corroded during the plating procedure to deteriorate its adhesion property and corrosion-resistant property.
Further, as to the latter spraying method, since the resin coating by this method has directionality, a great deal of working steps and time are required for applying the uniform resin coating over the entire surface of the workpiece to be treated; in particular, coating of a magnetic body having a complicated configuration with the coating film of a uniform thickness is all the more difficult. Furthermore, with the dipping method, thickness of the resin coating becomes non-uniform with the consequence that the finished product has a poor dimensional precision.
Furthermore, as the Fe-B-R type permanent magnet which could successfully solve the disadvantages inherent in the abovementioned plating method, spraying method and dipping method, and provide stabilized corrosion resistant property over a long period of time, there were also proposed improved permanent magnets provided on its surface with a vapor-deposited corrosion-resistant layer composed of various metals or alloys (Japanese Patent Applications No. 59-278489, No. 60-7949, No. 60-7950 and No. 60-7951, now corresponding EPA 0190461). By this vapor-deposition method, oxidation of the surface of the magnet body is suppressed, so that the magnetic property is prevented from deterioration. Also, since there is no necessity for use of corrosive chemicals, etc., hence no apprehension whatsoever of its remaining in the magnet body as it the case with the plating method, the permanent magnet as treated by this method is capable of retaining its stability over a long period of time.
While the vapor-deposition method is highly effective for improvement in the corrosion resistance of the permanent magnet, it has its own disadvantage such that a special treating apparatus is required, and its productivity is low, so that the treatment by this method is considerably expensive.
U.S. Pat. No. 4,588,439 discloses an Fe-B-R type permanent magnet alloy containing 6,000 to 35,000 ppm, (preferably 9,000 to 30,000 ppm) oxygen in order to avoid disintegration of the sintered body based on an autoclave test. However, this alloy consumes much rare earth elements as oxides. For complete suppression 9,000 ppm oxygen is necessary. Namely rare earth elements of 6 times by weight of the oxygen amount is consumed to form oxides. Such large amount of oxide is not preferred since the presence of nonmagnetic oxides adversely affects the magnetic properties, and valuable rare earth elements are consumed. For instances, 10,000 ppm oxygen will consume 6% by weight of rare earth elements as oxides.